Radiation therapy can be given to treat proliferative tissue disorders including but not limited to cancer, arteriovenous malformations, dermatological legions etc. During radiation therapy, the tissue of the patient known to or suspected to contain the disease is exposed to radiation. Linear accelerators are commonly used to irradiate a target volume encompassing the tissue to be treated during radiation therapy. As is known, linear accelerators use microwave technology to accelerate electrons in a waveguide and then allow the electrons to collide with a heavy metal target. As a result of the collisions, high-energy x-rays are scattered from the target. A portion of the scattered x-rays is collected and shaped by a beam collimating device to form an output beam of radiation conforming to the shape of the target volume. The linear accelerator also includes a gantry that rotates around the patient allowing the output beam of radiation to be delivered to the desired target volume from any angle by rotating the gantry.
Prior to exposing a patient to radiation, a treatment plan is typically developed in order to determine accurately the location of the tissue to be treated and how best to treat the tissue with radiation. Many imaging techniques have been used in treatment planning such as for example, computed tomography (CT), magnetic resonance imaging (MRI), and nuclear scintigraphy including single photon emission tomography (SPECT) and positron emission tomography (PET). Acquired images of the tissue are used to define the target volume so that the actual tissue irradiated by the output beam of radiation conforms as much as possible to the target volume. In many instances, the images of the tissue used to define the target volume are acquired in a single simulation.
For dose delivery, techniques such as tumour immobilisation with IMRT and image guidance have commonly been utilized. The purpose of image guidance is to ensure that the target tissue is placed at the isocenter of the linear accelerator at the beginning of radiation treatment. In tissue sites where a large amount of tissue motion is expected (for instance lung cancer radiotherapy), image guided therapy also constitutes control of the output beam of radiation to ensure that the irradiation time is restricted to the moment when the tissue is localized at the linear accelerator isocenter.
Unfortunately, this method has a fundamental difficulty if the image used to define the target volume is acquired in a single simulation since it is not known if image guided reproduction of the target location in subsequent treatment fractions results in the planned dosimetry being accurately delivered to the target and non-target tissues. This is because it is not known, a priori, if the single simulation image is representative of the patient positioning and target volume configuration in subsequent radiotherapy treatment fractions.
To provide more accurate position information concerning the target tissue and ensure the beam of radiation is properly directed in subsequent radiotherapy treatment fractions, it has been considered to integrate a linear accelerator with a magnetic resonance imaging apparatus.
MRI is a well-known imaging technique. During MRI, a target, typically a human patient, is placed into an MRI machine and subjected to a uniform magnetic field produced by a polarizing magnet housed within the MRI machine. Radio frequency (RF) pulses, generated by an RF coil housed within the MRI machine in accordance with a particular localization method, are used to scan target tissue of the patient. MRI signals are radiated by excited nuclei in the target tissue in the intervals between consecutive RF pulses and are sensed by the RF coil. During MRI signal sensing, gradient magnetic fields are switched rapidly to alter the uniform magnetic field at localized areas thereby allowing spatial localization of MRI signals radiated by selected slices of the target tissue. The sensed MRI signals are in turn digitized and processed to reconstruct images of the target tissue slices using one of many known techniques.
Unfortunately, even in such systems which use inter-fraction image guidance, tissue motion during the treatment fraction results in the delivered dose of radiation differing from the planned dose, and necessitates the use of a planning target volume (PTV) to ensure tissue coverage. The consequence is that the normal tissue surrounding the target tissue also receives an additional radiation dose.
Further, real-time, three-dimensional (3D) imaging of the patient during the radiation therapy has not been incorporated into IMRT delivery devices, and so it does not allow for true radiation dose reconstruction post treatment. The current art only calculates a post-treatment radiation dose reconstruction based on a pre-treatment static image of the target volume.
In addition, while it is desirable to have real-time imaging during radiotherapy, the true quantity of interest is the real-time accumulation of radiation doses to all structures in the target volume. Real-time dose-accumulation data could be used to detect treatment errors at a very early time, and would allow intervention in the instance where the treatment fraction dose is outside of tolerance. This is not currently available since no method of radiation dose calculation is currently fast enough (unless prohibitively large numbers of parallel processing computers are used) to perform the necessary dose accumulation calculations in real-time. As well, the availability of real-time radiation dose calculation would allow safe dose escalation in those tissue sites where the optimal dose has not yet been defined.
As will be appreciated, improved radiation therapy techniques which obviate or mitigate one or more of the above-referenced problems are desired. It is therefore an object of the present invention to provide a novel real-time dose reconstruction using dynamic simulation and image guided adaptive radiotherapy.